The present invention relates to a method for the preparation of 4,6-dichloropyrimidine from 4-chloro-6-methoxypyrimidine. 4,6-Dichloropyrimidine is a valuable intermediate for the preparation of crop protection agents.
A number of methods for preparing 4,6-dichloropyrimidine are known, see, for example, WO96/23776, EP-A-697 406, EP-A-745 593, WO 95/29166, DE-A-195 31 299 and GB 2 325 224. However, these methods always start from 4,6-dihydroxypyrimidine.
It is also known (see Res. Discl. n 391, 690-691 (1996)), that 4,6-dichloropyrimidine can be obtained by reaction of 4-chloro-6-methoxypyrimidine with a chlorinating reagent of the formula R3PCl2. The chlorinating reagent can be employed as such or be prepared in situ from a compound of the formula R3P=O and phosgene. This reference suggests that 4-chloro-6-methoxypyrimidine does not react with phosgene alone in the desired manner.
A method for the preparation of 4,6-dichloropyrimidine has now been found and is characterized in that 4-chloro-6-methoxypyrimidine is reacted with phosgene as chlorinating agent in the presence of nitrogen-containing auxiliaries.
The method according to the invention can be carried out in a solvent (for details, see below) or in the melt. The procedure in a solvent is preferred.
Examples of suitable nitrogen-containing auxiliaries are nitrogen-containing bases, for example amines of the formula R1R2R3N (in which R1, R2 and R3 can denote independently of one another in each case for C1-C10-alkyl, C6-C10-aryl, C5-C9-heteroaryl with 1 to 3 heteroatoms from the group of N, O and S or C6-C10-aryl-C1-C6-alkyl) or unsaturated or saturated cyclic amines with 1 to 2 nitrogen atoms and from 5 to 11 carbon atoms, it being possible for the cyclic amines optionally to be substituted once to 3 times by C1-C10-alkyl. Examples of such amines are: triethylamine, N,N-diethylaniline, N,N-dimethylaniline, diisopropylethylamine, 4-(N,N-dimethylamino)pyridine (DMAP), C1-C2-alkyl-mono- or -dialkylated pyridines, morpholine, imidazole, triazole, 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and piperidine. It is furthermore possible to employ as nitrogen-containing auxiliaries amides and ureas, which can also be used as solvents. Examples are: N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, tetramethylurea and cyclic ureas such as 1,3-dimethyimidazolidin-2-one (DMEU) and 1,3-dimethyltetrahydro-2(1H)-pyrimidinone (DMPH).
In general, at least 1 mol of phosgene is employed per mole of 4-chloro-6-methoxypyrimidine in the method according to the invention. This amount is preferably 1.05 to 20 mol, particularly preferably 1.1 to 10 mol.
The amount of nitrogen-containing auxiliaries can be varied within a wide range. Smaller amounts, for example those below 1 mol per mole of 4-chloro-6-methoxypyrimidine can be employed, for example, if the nitrogen-containing auxiliary is intended to be used as catalyst. Larger amounts, for example those of more than 1.5 mol per mole of 4-chloro-6-methoxypyrimidine can be employed if it is wished to employ the nitrogen-containing auxiliary both as catalyst and as solvent. For example, the amount of nitrogen-containing auxiliary can be between 0.001 and 25 mol, preferably between 0.01 and 15 mol, in each case based on 4-chloro-6-methoxypyrimidine. Amounts in the range from 0.01 to 0.5 mol per mole of 4-chloro-6-methoxypyrimidine are particularly preferred on use with the nitrogen-containing auxiliary as catalyst.
If it is wished to have solvents present, solvents which are suitable in principle are those which do not have adverse effects on the reaction to be carried out. Examples are aliphatic solvents such as alkanes, cycloalkanes and halogenoalkanes, aromatic solvents such as benzene, xylenes, toluene, chlorobenzenes, benzotrifluorides, p-chlorobenzotrifluoride and anisole, it being possible for the aliphatic and aromatic solvents to be further substituted where appropriate, nitriles such as acetonitrile and benzonitrile, N-containing solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, lactams and cyclic ureas and ethers and polyethers of a wide variety of types.
The method according to the invention can be carried out for example at temperatures in the range from 0 to 200xc2x0 C., preferably at 20 to 150xc2x0 C., particularly preferably at 40 to 120xc2x0 C. The pressure is not critical. It is possible to operate, for example, at 0.1 to 50 bar, preferably at 0.5 to 5 bar. Atmospheric pressure is particularly preferred.
The method according to the invention can be carried out in various embodiments, for example batchwise, semibatchwise, semicontinuously or continuously. One possible procedure is as follows: gaseous phosgene is passed into 4-chloro-6-methoxypyrimidine mixed with a nitrogen-containing auxiliary, where appropriate together with a solvent.
Another variant is to add phosgene in liquid form or dissolved in a solvent. It is moreover possible to add all the phosgene immediately at the start of the reaction or meter it distributed over a certain period.
The reaction mixture present after the reaction can be worked up, for example, by initially removing excess phosgene from the mixture by blowing out and/or partial distillation, and distilling the remaining reaction mixture.
If water-soluble auxiliaries have been employed, it is beneficial first to add water to the reaction mixture and, after the auxiliaries have been washed out and after the solvent has been distilled out, to distill or crystallize the remaining product.
Another generally advantageous variant consists of working up by extraction. On suitable choice of the combination of nitrogen-containing auxiliary with the solvent, in the simplest case N,N-dimethylformamide as nitrogen-containing auxiliary and xylene as solvent, the reaction mixture separates into two phases. The 4,6-dichloropyrimidine-containing xylene phase can then be separated off, and the N,N-dimethylformamide phase can be extracted one or more times more with xylene. The combined xylene phases can then be distilled.
It is also possible to carry out the reaction according to the invention only in the presence of a nitrogen-containing auxiliary and then to extract the resulting reaction mixture with a suitable solvent, for example aliphatic or aromatic hydrocarbons such as hexane, isooctane, methylcyclohexane, toluene, xylene, decalin, tetralin or hydrocarbon mixtures.
The method according to the invention is exceptionally surprising in the light of the reference Res. Discl. loc. cit. Although phosgene is mentioned as chlorinating agent therein, it is only for in situ generation of triorganodichlorophosphorane. Direct reaction of 4-chloro-6-methoxypyrimidine with phosgene is not mentioned.
The method according to the invention allows 4,6-dichloropyrimidine to be prepared in a simple manner and in good yields and without the use of phosphorus-containing chlorinating agents.
The following examples further illustrate details for the process of this invention. The invention, which is set forth in the foregoing disclosure, is not to be limited either in spirit or scope by these examples. Those skilled in the art will readily understand that known variations of the conditions of the following procedures can be used.